Thunder’s “five-second mile”
Counting five seconds between flash and boom means the storm is about one mile away. In kilometers, think “three seconds per kilometer” — the rule of thumb this tool shows.
Storm gauge
Thunder Distance Calculator
Quick rule of thumb
Distance is about seconds ÷ 5 miles or seconds ÷ 3 kilometers.
Use this for a fast thunder estimate. The calculator below uses temperature-adjusted sound speed for the detailed result.
Thunder and sound calculator
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Results
Estimated lightning distance
–
Enter seconds after lightning to estimate the distance.
Miles–
Kilometers–
Feet–
Meters–
Seconds–
Speed, m/s–
Speed, ft/s–
Speed, mph / km/h–
If you can hear thunder, move indoors or stay in a safe shelter.
Assumes still air and dry air speed c = 331.3 × sqrt(1 + T/273.15) m/s.
Light (flash) is instant for our purposes; the bar shows when the sound arrives.
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Echo calculator
For echoes, sound travels to a wall or cliff and back, so the time is a round trip.
Common thunder delays and sound travel examples
Values use 20 °C air. Warmer or colder air changes them slightly.
| Example | Result at 20 °C | Useful reading |
|---|---|---|
| 1 second after lightning | 0.21 mi / 0.34 km | Very close |
| 3 seconds after lightning | 0.64 mi / 1.03 km | About 1 km |
| 5 seconds after lightning | 1.07 mi / 1.72 km | About 1 mile |
| 10 seconds after lightning | 2.13 mi / 3.43 km | Distant, but still audible |
| 15 seconds after lightning | 3.20 mi / 5.15 km | Rule of thumb: about 3 miles |
| 30 seconds after lightning | 6.40 mi / 10.30 km | Thunder can still signal risk |
| 1 km sound travel | 2.91 seconds | Close to the 3 seconds per km rule |
| 1 mile sound travel | 4.69 seconds | Close to the 5 seconds per mile rule |
| 100 m echo | 0.58 seconds round trip | Echo distance is doubled |
Learn more: how sound travel time works
When you see lightning and then hear thunder later, you’re watching physics in action. Light races to your eyes almost instantly, but sound has to push through the air molecule by molecule. That “push” travels at the speed of sound, which depends mostly on air temperature. Warmer air lets molecules jostle faster, so sound travels faster; colder air slows things down.
Speed of sound formula
This calculator uses the standard dry-air ideal-gas approximation for temperature in Celsius:
c = 331.3 × sqrt(1 + T / 273.15) (meters per second)
At 20 °C this gives about 343.2 m/s. That turns into two handy rules of thumb: roughly 3 seconds per kilometer and roughly 5 seconds per mile. If you count 9 seconds between the flash and the boom on a mild day, the storm is around 3 km away (or ~1.8 mi).
Distance, time, and echoes
The relationships are simple:
time = distance ÷ c distance = time × c echo_time = 2 × distance ÷ c
For echoes, your voice goes out to a wall or cliff and returns to you, so the timing is a round trip. If a canyon wall is 300 m away and the air is mild, the echo comes back in well under two seconds. Big outdoor spaces (stadiums, valleys, canyons) make great “echo labs” because the geometry is large enough to hear a clear delay.
What else changes the number?
- Temperature: the biggest factor. Warmer → faster sound → shorter delay.
- Humidity: moist air is slightly faster than dry air, but the effect is small.
- Altitude & pressure: thinner air nudges the speed a bit; our estimate keeps it simple.
- Wind: can shave or add a fraction of a second depending on direction.
- Terrain & surfaces (echoes): soft, uneven surfaces absorb and scatter sound; hard, flat rock reflects crisply.
Why thunder sounds “rumbly”
Lightning is a long, branching path. Different parts of that path are different distances from you, so sound from some parts arrives earlier and some later. The first crack is usually from the nearest part of the bolt; the low rumble is all the slightly farther segments arriving afterward and bouncing around the landscape.
Safe, sensible use
This calculator is built for friendly estimates and classroom demonstrations. Treat the results as approximations, not professional safety advice. If the delay is small, the storm is close — head indoors and follow local guidance. For echoes, remember that not every location will produce a strong reflection even if the math says when it would return.
Try these fun experiments
- Count & compare: On a cool day vs. a hot day, count seconds after a distant slam (or a safe, known sound) and compare your calculated distances.
- Echo hunt: Clap near a large building, a gym, and a rocky outcrop. Which one gives the clearest echo? Measure the delay with your phone’s voice memo timeline.
- Thumb rules check: Does “3 s/km” or “5 s/mi” match your results at today’s temperature?
Behind the scenes, this page runs 100% in your browser and doesn’t store your inputs. It uses the temperature-based formula above to keep the calculation transparent and close to what you’ll observe outdoors.
Methodology and trust notes
Formula: c = 331.3 × sqrt(1 + T/273.15) m/s, where T is air temperature in °C. Distance = delay × c. Sound travel time = distance ÷ c. Echo time = 2 × distance ÷ c.
Assumptions: dry air, still air, direct sound path, and ordinary outdoor temperatures. Humidity, wind, terrain, altitude, and the shape of the lightning channel can shift real thunder timing.
References: National Weather Service pages on thunder and lightning distance and lightning safety; standard ideal-gas speed-of-sound relationship for dry air.
Last updated: June 29, 2026. Privacy: the calculation runs in your browser and does not upload your inputs.
Validation examples: at 20 °C, 5 s is about 1.07 mi / 1.72 km; 10 s is about 2.13 mi / 3.43 km; 1 km takes about 2.91 s.
5 Fun Facts about Sound Travel Time
Sound isn’t fastest when it’s hot
A 10 °C swing changes speed by ~6 m/s. Going from freezing to a hot summer day speeds sound up enough to shave almost half a second off a 1 km thunder delay.
Temperature lever
Lightning is an audio slideshow
Bolts can stretch for kilometers. You hear the closest segments first (sharp crack) and the distant branches later (low rumble). Thunder is really a layered soundtrack.
Boom anatomy
Echoes double the distance
An echo time is a round trip. If your shout comes back in 0.8 s, the cliff is ~140 m away at mild temperatures—half the distance your sound actually travelled.
Canyon math
You can “see” distance with sound
Blind people and animals like bats use echoes to map space. Their brains time tiny delays — down to a few milliseconds — to sense shapes. This calculator is the simplified version you can try outside.
Echo sensing
Thunder distance FAQ
How far away is lightning after 5 seconds?
A 5 second delay is about 1 mile or 1.7 kilometers away at 20 °C. The quick rule is seconds divided by 5 for miles.
How far away is thunder after 10 seconds?
A 10 second delay is about 2.1 miles or 3.4 kilometers away at 20 °C.
How many seconds per mile?
Use about 5 seconds per mile. With 20 °C air, one mile takes about 4.7 seconds.
How many seconds per kilometer?
Use about 3 seconds per kilometer. With 20 °C air, one kilometer takes about 2.9 seconds.
What is the speed of sound at 20 °C?
Using c = 331.3 × sqrt(1 + T/273.15), the speed of sound at 20 °C is about 343.2 m/s, 1,236 km/h, or 768 mph.
Does temperature affect thunder distance?
Yes. Warmer air carries sound slightly faster, so the same delay maps to a slightly larger distance.
Does humidity or wind matter?
Humidity and wind can nudge the result, but this calculator keeps the estimate simple by adjusting for temperature and assuming still air.
Is it safe if I can hear thunder?
No outdoor location is considered safe when thunder is audible. Move indoors or remain in a safe shelter.
Notes & safety
- Thunder & lightning safety: if the delay is small, the storm is close — go indoors.
- Wind, humidity, and altitude nudge the numbers; temperature is the big factor.
- Echo strength depends on the surface; some spots won’t echo even if the math says when it would arrive.
- No data leaves your device; everything is computed locally.